In the semiconductor technology, the materials, such as silicon dioxide(SiO2), silicon nitride (Si3N4), polysilicon, silicon, aluminium and other materials, have to be etched in order to provide a structured substrate with can be processed further. The purpose of etching of the substrate is to remove one or more materials or to transfer the lithographic pattern into the substrate layers. The etching step should take place selectively, i.e., only one material should be etched, even though a plurality of materials may be present.
There are various methods which can be used for etching different materials, but all the etching methods can be divided in one of the two categories, namely either dry etching or wet etching processes.
In wet etching processes, the solid material is dissolved into the solution by applying an acidic or a basic solution. An etching solution can react either isotropically or anisotropically with the substrate. An isotropic etching solution etches the material in all directions, whereby an anisotropic etching solution etches a material only in one direction, namely 90 degrees to the substrate surface. There are many solutions which can be used in the semiconductor processing, which differ as to the concentration and to the composition of the materials which can be used on one side and to the materials which are to be etched on the other.
For example, for the silicon etch, there are solutions which can etch silicon isotropically and some which have preference to one of the crystal layers of the silicon crystal. Only when there is a preference for one of the layers, the anistropic etch can take place. One of the solution which can particulary be used for the silicon etch is tetra methyl ammonium hydroxide (TMAH), but this solution has very low selectivity for the different crystal layers of silicon.
In order to etch silicon anisotropically, there are several solutions which can be used. The most used solution are basic, and are, for example, KOH, NaOH, LiOH, or a mixture of ethylene diamine with other components in water (EDP-solution).
When silicon and silicon oxide are present on a substrate, and when both materials should be etched, wherein silicon is to be etched anisotropically, a two step process is usually used.
In the first step, the silicon layer is etched by solutions like KOH, NH4OH, TMAH, and choline. These solutions have high silicon etch rates and high selectivity to the oxide. For example, silicon oxide is, in the instant case, not etched at all. In the second step the silicon dioxide layer is etched using HF, which is usually present in a buffered solution, together with NH4F. In the semiconductor technology, there are various methods to prepare a layer of silicon dioxide. For example, the silicon dioxide layer can be prepared by thermal oxidation of silicon (thermal oxide) or by depositing silicon dioxide by using, for example, a chemical vapour deposition (CVD) process. Depending on the method of preparation of the oxide layers, these layers differ in chemical and mechanical properties. A thermal oxide can, for example, be etched with a solution containing NH4F/HF/H2O in a relationship 2:1:7 with an etch rate of 50 nm/min., and TEOS-oxide can be etched with the same solution with a higher rate of about 150 nm/min. The solution containg NH4F/HF/H2O is known to etch silicon oxide selective to silicon with a selectivity ratio of 100:1. This means that the silicon dioxide layer is etched 100 faster than the silicon layer.
However, the application of the chemicals in the two-step process described above is possible only in covered equipments (point of use tasks) due to an initial HF-step to remove native or thermal oxide. Furthermore, a two chemical tank sequence with a DHF, rinse, NH4OH, rinse, dry shows problems regarding wettability and reaction of the forming gas (H2). In the etching processes no gas should be formed, since the produced bubbles can case uneven etching of the material.
A method for etching both silicon oxide and silicon layers simultaneously, where the silicon layer is etched anisotropically, is desirable.